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Full text of "Handbook Of Chemical Engineering - I"

EVAPORATION
365
the temperature difference between the two evaporators, and these temperatures cannot be determined exactly beforehand. The case becomes even more complicated if the condensate of the second effect is discharged into the steam chest of the third effect to save the heat of reevaporation. E. Hausbrand, in his book, "Evaporating, Condensing and Cooling," has given some excellent formulas and tables for various temperatures and percentages of concentration taking into account the amount of heat
W-
°i                              C2
FIG. 4.—Heat balance in multiple effect.
H
recovered by reevaporation of the liquor passing from one body to the next with the assumption that the condensate is discharged from each steam chest separately. The average results are:
Double effect;        Vi = 0.466; 72 = 0.534 Triple effect;         7i = 0.300; 72 = 0.329; 73 = 0.371
Quadruple effect; 7i = 0.216; 72 = 0.243; 73 = 0.256; 74 = 0.285 where 7i, 72, 73, 74 are the amounts of evaporation in each effect, and the sum is the difference between the weight of the ingoing and outgoing liquors. These percentages are correct for a concentration of the liquor of 7 to 1, and 7i will be about 8 per cent smaller for a concentration of 10 to 1, and 5 per cent larger for a concentration of 4 to 1. All these figures are based on the assumption that the liquor enters at the temperature prevailing in the first effect and allowances must of course be made if the liquor is warmer or colder.
In practically all cases, the condensate coming from the steam chest of the first effect is returned to the boiler; in a triple or quadruple effect a slight saving in steam may be accomplished by discharging the condensed water from the second steam chest into the third and from the third into the fourth steam chest. For average conditions the factors of distribution in this case would be:
Triple effect; V1 = 0.295; 72 = 0.324; 73 = 0.381 Quadruple effect; Vl = 0.211; 72 = 0.238; 73 = 0.256; 74 = 0.295 The steam consumption will also depend on the amount of heat lost by radiation-For evaporators not covered by insulating material, the loss R will be 3 B.t.u. per degree Fahrenheit per hour per square foot of outside surface, and 0.5 B.t.u. per degree Fahrenheit per hour per square foot for apparatus that arc well covered with magnesia or corrugated asbestos paper.    The degrees Fahrenheit arc equal to the